Fluidized bed combustion reactors are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur released as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed reactor offers an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed reactor includes what is commonly referred to as a bubbling fluidized bed in which a bed of particulate material is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. In the event the reactor is in the form of a steam generator, the walls of the reactor are formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to a turbine to generate electricity or to a steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a fast fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, with the air velocity equal to or greater than that of a bubbling bed. The formation of the low density fast fluidized bed is due to its small particle size and to a high solids throughput, which requires high solids recycle.
The high solids circulation required by the fast fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the combustor or gasifier, and therefore decreasing the nitrogen oxides formation. Also, the high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. Furthermore, the fast fluidized bed inherently has more turn-down than the bubbling fluidized bed.
However, the fast fluidized bed process is not without problems. For example, the particulate fuel and adsorbent material used in a fast fluidized bed process must be relatively fine therefore requiring further crushing and drying of the particulate material, which is expensive. Also, the bed height required for adequate adsorption of the sulfur will be greater than that in a conventional bubbling fluidized bed system, which further adds to the capital expense and operating costs.
A reactor of the type disclosed in U.S. Pat. No. 4,809,623 incorporates operating principles and advantages of both the bubbling fluidized bed and the fast fluidized bed. The "hybrid" reactor and method features the forming of a gas column above a fluidized bed which contains a mixture of air, the gaseous products of combustion from a fluidized bed and particulate material, a portion of which is coarse enough to continuously stay in bed, while the rest is fine enough to be entrained by the air and the gaseous products of combustion. The gas column is saturated with particulate material and the particulate material is separated from the mixture and a portion of the separated particulate material is passed to external equipment.
Bubbling fluidized bed and especially the fast or hybrid fluidized bed combustion reactors require relatively large cyclone separators for the separation of entrained solid particles from the combustion gases and for solids recycle. A typical cyclone separator includes a vertically oriented, cylindrical vortex chamber in which is disposed a central gas outlet pipe for carrying the separated gases upwardly, while the separated particles are returned to the bed through a funnel-shaped base of the separator via a standpipe. These so-called vertical cyclone separators are substantial in size and eliminate the possibility of a compact system design which can be modularized and easily transported and erected. For larger combustion systems, several vertical cyclone separators are often required to provide adequate particle separation, which compound the size problem and, in addition, usually require complicated gas duct arrangements with reduced operating efficiency.
Horizontal cyclone separators characterized by a horizontally-oriented vortex chamber have been constructed which eliminate many of the above mentioned problems. For example, horizontal cyclone separators may be readily configured within the upper portion of the reactor and integrated with the walls of the reactor. However, known horizontal cyclone separators have various shortcomings, particularly with regard to their circulation and gas discharge arrangements and require extended ducting for transfer of the gases to a heat recovery area, and are otherwise less efficient in their construction and/or operation.